David Cameron has mounted a strong defence of his decision to travel to Japan with some of Britain's leading weapons manufacturers as Downing Street seeks to exploit a multibillion-pound market after a liberalising of Tokyo's procurement rules.

As he flew to Japan overnight, the prime minister said he was "up front" about the "perfectly responsible and respectable" decision to travel with executives from six defence contractors, including BAE Systems and AgustaWestland.

Britain is preparing to embark on developing weapons jointly with Japan after Tokyo named the UK as its first overseas defence trading partner after the US.

Speaking on board his chartered plane, the prime minister said: "There are a number of defence manufacturers with us. I'm completely up front about that because we do have a very strong defence sector. It accounts for a lot of jobs, we have some of the toughest rules on defence exports anywhere in the world.

"But as these countries, particularly Japan, that have tended in the past to buy only American equipment are opening up, there are opportunities for people like AgustaWestland, who make helicopters, who are on this plane. I think that's perfectly responsible and respectable."

Downing Street – acutely sensitive to charges that the prime minister drums up business for defence manufacturers on his overseas tours – will hope that the focus of Tuesday's visit will be on Nissan's Yokohama headquarters. After landing at Tokyo's Haneda airport at lunchtime Japanese time, the prime minister headed to the plant where the carmaker will announce a £127m investment in its Sunderland plant to produce its new hatchback, expected to create 225 jobs.

Cameron said the Nissan announcement, the first of a series of deals in Japan and in countries in south-east Asia he will visit this week, showed the importance of his trade mission. The prime minister, who is being accompanied by 33 British business leaders and representatives of four universities, said: "Part of the job of the prime minister is to load up an aeroplane full of business people, large and small, get our exports up, get our investment up, get out there and fly the flag for Britain. That is what I am trying to do this week."

The prime minister added: "It is an important trip. Obviously these are key diplomatic relations with Japan and south-east Asia that we want to build. We think in some ways we have underplayed these relations in the past. We want to boost them and strengthen them. Above all this trip is really about British business. We need to rebalance the British economy, we need to grow our exports, we need to grow our investment, we need to grow our manufacturing and our industries. All these countries represent huge opportunities."

But Cameron's talks in Tokyo with Yoshihiko Noda, his Japanese counterpart, will be dominated by expected deals on defence co-operation and Britain's role in helping Japan with its nuclear decommissioning. These could eventually dwarf the relatively modest Japanese inward investment deals, worth £200m, that will be signed on the trip.

The prime minister is taking representatives of the defence manufacturers, which also include Babcock, MBDA, Rolls Royce and Thales, because officials believe there is a major opportunity as Japan opens up its multibillion-pound defence market. Britain hopes to win a share of the market, which had been open only to Japanese and US companies since 1967.

A blanket ban came into force in 1976, although this did not apply to the US.

In December last year Japan lifted the ban – a move technically outside the terms of the postwar constitution, enacted in 1947, which banned "land, sea, and air forces, as well as other war potential". Japan has traditionally circumvented the ban by building up its armed forces with the help of the most formidable of the allied powers in 1945, the US.

The Mainichi newspaper last week quoted defence ministry officials in Tokyo as saying that Cameron and Noda would formally agree to begin defence co-operation talks. The officials said it could take a year to decide on areas where the two countries would co-operate, but they are expected to start initially with the joint development of military equipment.

Britain says progress has already been made in highly technical preliminary negotiations. This has involved persuading the Japanese that UK defence equipment is "interoperable" and meets the security requirements of both sides.

Cameron will work hard to press British interests after Japan recently chose US-made Lockheed Martin F-35 fighter jets over the Eurofighter Typhoon manufactured by a consortium of European companies, including BAE Systems.

Cameron faced embarrassment in February last year over the promotion of Britain's defence industry when he took a delegation of manufacturers to the Gulf. He began the trip by hailing democracy in Cairo's Tahrir Square – before flying to Kuwait with eight defence manufacturers. The prime minister will argue that this trip is different because Japan is a democracy.

Downing Street believes in recent years Britain has overlooked its relationship with Japan, which is the world's third largest economy and whose investments in the UK come to more than £26bn, accounting for 130,000 jobs. Japan is only the 14th largest export market for UK goods, though this grew by 7% last year.

Tony Blair paid the last bilateral visit by a British prime minister to Japan in 2003. Cameron was due to visit Japan last October on his way to the Commonwealth heads of government meeting in Australia. But by then he had already visited China, India and the US.

In an interview with the Japanese daily Yomiuri, the world's largest newspaper, Cameron said he hoped to revive ties with an old ally. "We do have very good relations between Britain and Japan. We're old friends, we're old allies, we're partners in many senses. But I think we can take the relationship even further."

Cameron paid tribute to the way the Japanese people had responded to the twin disasters of the tsunami and the subsequent nuclear disaster at Fukushima last year. "I greatly admire and respect the way the Japanese have overcome the enormous challenges of recovery following last year's earthquake. The UK stood by Japan in the immediate aftermath of the earthquake when we kept our embassy open in Tokyo and through the reassuring advice of our chief scientific adviser, Sir John Beddington, who will be joining me on this trip.

"Looking to the future, we want to continue to support the Japanese as they reconstruct the Tohoku region and tackle the challenges of the nuclear clean-up. British companies have significant expertise in nuclear decommissioning and clean-up, with 19 nuclear sites in the UK currently being managed through the process."